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Here is salient commentary on register windows, and other bad ideas from early RISC (from the perspective of debugging assembly language). https://www.jwhitham

by chasil 4y ago

Here is salient commentary on register windows, and other bad ideas from early RISC (from the perspective of debugging assembly language). https://www.jwhitham.org/2016/02/risc-instruction-sets-i-have-known-and.html https://www.jwhitham.org/2016/02/risc-instruction-sets-i-hav...

gumby 4y ago
A good post, though I have a couple of minor criticisms: > Nobody writes assembly code any more, except when they do, His very first sentence turns out to be the primary assumption of RISC: essentially nobody writes assembly code so don't worry about making that easy, and depend on the HLL compiler to do a lot of heavy lifting. The small amount of assembly is just to boot the processor, boot a process, and some small glue in the OS, and as that stuff is heavily used but almost never written you don't have to worry about alleviating those peoples' suffering. Oh, and compiler and debugger writers, and he does point out how some of these decisions make things more complex for them! > CISC had always involved decoding instructions into micro-instructions. That isn't actually true; into the 60s instructions were implemented in hardware, and the roots of CISC lie there -- essentially some quintessentially CISC instructions (BCD support, string handling, etc) were subroutines implemented in hardware because they were so common and that made the computer easier to sell. (as an aside: back in those days, when the machines had high level languages they were often unique to the vendor or even the specific machine model!) Then again, even tiny machines like the 8080 that weren't really RISC or CISC had microcode, but that was later.
- userbinator 4y ago
  The 8080 has nothing resembling microcode, and neither do the 8085 nor Z80. They're all based on an instruction decode PLA with a simple sequencer/state machine logic. Perhaps you're thinking of the 8086?
  - gumby 4y ago
    Yes that was a typo (or thinko), perhaps because I was discussing this very topic with Stan Mazor the day before yesterday! Thanks.
moremetadata 4y ago
[dead]
speed_spread 4y ago
I was reading today about AMD 29000 and it came to my attention that a register windows wasn't a such a bad idea but having a fixed window size (as SPARC did) definitely was.

by chasil 4y ago

gumby 4y ago

A good post, though I have a couple of minor criticisms: > Nobody writes assembly code any more, except when they do, His very first sentence turns out to be the primary assumption of RISC: essentially nobody writes assembly code so don't worry about making that easy, and depend on the HLL compiler to do a lot of heavy lifting. The small amount of assembly is just to boot the processor, boot a process, and some small glue in the OS, and as that stuff is heavily used but almost never written you don't have to worry about alleviating those peoples' suffering. Oh, and compiler and debugger writers, and he does point out how some of these decisions make things more complex for them! > CISC had always involved decoding instructions into micro-instructions. That isn't actually true; into the 60s instructions were implemented in hardware, and the roots of CISC lie there -- essentially some quintessentially CISC instructions (BCD support, string handling, etc) were subroutines implemented in hardware because they were so common and that made the computer easier to sell. (as an aside: back in those days, when the machines had high level languages they were often unique to the vendor or even the specific machine model!) Then again, even tiny machines like the 8080 that weren't really RISC or CISC had microcode, but that was later.

userbinator 4y ago
The 8080 has nothing resembling microcode, and neither do the 8085 nor Z80. They're all based on an instruction decode PLA with a simple sequencer/state machine logic. Perhaps you're thinking of the 8086?
- gumby 4y ago
  Yes that was a typo (or thinko), perhaps because I was discussing this very topic with Stan Mazor the day before yesterday! Thanks.

moremetadata 4y ago

[dead]

speed_spread 4y ago

I was reading today about AMD 29000 and it came to my attention that a register windows wasn't a such a bad idea but having a fixed window size (as SPARC did) definitely was.